Owing to known strong homopolymerization tendency of acrylic monomers in order to yield polymers comprising acrylic unsaturations, relatively low temperatures are used (in the range of 60 to 130xc2x0 C.; at most 150xc2x0 C.) together with suitable amounts of polymerization inhibitors and by adding solvent in order to lower polymer viscosity. In this way, uniform mass heating is favoured and local superheating, which would cause the polymerization of unsaturated functionalities, is avoided.
Several processes, working either in the presence of solvents or at relatively low temperatures together with short reaction times to yield polymers comprising unsaturated groups for powder paints, are known.
EP 0249306 (Autotype Int.Appl.)discloses an unsaturated polymer obtained in toluene solution wherefrom it is precipitated and filtered. EP 0636669 and EP 0644905 (DSM) disclose polymers obtained at temperatures of 50 to 110xc2x0 C. in butylacetate solution or toluenetxylene solution followed by solvent stripping at 150xc2x0 C. EP 0650978 (BASF) discloses unsaturated polymers obtained without solvents, but homogenized by extruding reactive compounds together with unsaturated functional monomers at temperatures up to 130xc2x0 C. for a short time (7-15 min).
These limitations in temperatures and time impair the possibility of using the synthesis. However, many manufactures of resins for powder paint are in the need of carrying on, also for these types of resins, the use of technologies and work cycles used for conventional resins such as polyesters and acrylics, which are manufactured at temperatures above 150xc2x0 C., up to 260xc2x0 C., and unloaded at temperatures between 160 and 220xc2x0 C. for a relatively long time (0.5-8 hours) dependent upon the resin amount and the crushing rate of the flaking machine. However, for the above stated reasons, that has not been possible up to now.
It has now surprisingly been found possible to add an unsaturated functional monomer to a resin comprising reactive functional groups, maintained at temperatures in the range of 150xc2x0 C. to 200xc2x0 C. and with an addition of suitable inhibitor amounts, and to react it with the resin for 50 to 60 minutes without affecting the resin unsaturation.
This object is achieved by supplying the reaction mass with heat in order to avoid local superheating. This in practice occurs by maintaining the temperature in the range of values reached during the exothermic reaction or slightly higher without the need of adding solvents.
Starting polymers usable for the purposes of this invention may be either acrylic resins or saturated or unsaturated polyesters having reactive groups such as hydroxy, carboxylic, glycidyl, isocyanate groups, whereas (meth)acrylics comprising functional groups mentioned above are especially used as reactive monomers.
The unsaturated functional monomers are preferably selected from the group consisting of glycidyl(meth)acrylate, (meth)acrylic acid, methacrylisocyanate, benzo-1-(1-isocyanate-1-methyl)-4,1-methenyl, isocyanateethylmethacrylate or unsaturated isocyanate adducts.
Polymerization inhibitors are conventional inhibitors such as hydroquinone, hydroquinone-monomethylether, mono-ter-butylhydroquinone, phenotiazine, diethylhydroxylamine, diphenyl-p-phenylenediamine,isopropyl-phenyl-p-phenylenediamine, triphenylphosphite.